Among optically transparent or semitransparent inorganic porous materials, there are known aerogels obtainable, for example by subjecting an alkoxysilane to hydrolysis and polycondensation and then drying the thus-formed gel-like compound (wet gel) by eliminating the solvent used in forming that gel-like compound. These aerogels are light and have good heat insulating properties and, therefore, have attracted attention as useful heat insulating materials in various fields.
In producing such aerogels, it is necessary to efficiently remove the solvent used in forming the above-mentioned gel-like compound (wet gel) from the gel-like compound to give a dry gel. However, if such a wet gel is dried as it is, the structure of the gel-like compound will be destroyed and the gel cannot be dried with the shape thereof being retained. This is considered to be due to the stress caused by the surface tension generated at the gas-liquid interface, among others.
As means for preventing such a structure destruction, a method is known as typical one which comprises drying in a supercritical condition (e.g. Non-Patent Document 1). Such a method is based on the principle that when the atmosphere is in a supercritical condition, there is no distinction between gas and liquid any longer, hence, theoretically, the surface tension generation at the gas-liquid interface can never happen.
As another technology utilizing such a principle, the technology described in Patent Document 1, for instance, has also been proposed. This technology teaches a method which comprises drying a gel-like compound while replacing the solvent (e.g. alcohol) contained in that compound with supercritical carbon dioxide. According to this method, a supercritical condition can be realized relatively easily and inexpensively since the critical temperature and critical pressure of carbon dioxide are relatively low.
However, a problem still remains. Namely, when the replacement of the gel-surrounding solvent as attainable by feeding carbon dioxide in a supercritical condition occurs at an excessively high rate, the stress increases and causes cracking of the aerogel as a result of interfacial tension generation, among others, within the wet gel due to rapid changes in concentration within the gel and so forth and as a result of diffusion and expansion of the solvent. Accordingly, for example, a measure is taken so that carbon dioxide in a supercritical condition may be fed over a long period of time.
On the other hand, from the cracking prevention viewpoint, such a technology as disclosed in Patent Document 2 has also been proposed. This technology consists in maintaining the high pressure vessel inside after solvent replacement at a temperature not lower than the boiling point of the solvent to prevent the solvent remaining in the atmosphere from liquefying and thereby prevent the solvent from generating a gas-liquid interface, thus preventing the gel-like compound structure from being destroyed. However, even when such a measure is taken, it cannot be said that the operation efficiency be much improved; and, when the rate of replacement is excessively high, the problem of cracking still remains.    Non-Patent Document 1: “Sol-Gel Ho no Kagaku (Science of Sol-Gel Method)”, published May 1, 1990 by Agne Shofusha, page 37    Patent Document 1: U.S. Pat. No. 4,610,863    Patent Document 2: Japanese Patent No. 3,048,276